Polyvinyl alcohol purification process

ABSTRACT

A poly(vinyl alcohol) purification process is described, which includes heat treating a poly(vinyl alcohol) under agitation in an atmosphere unsaturated by water and with a water vapor mass content of 20 to 100%. The process of the invention may also include an additional drying step. A poly(vinyl alcohol) with an alcohol content, more particularly a methanol content, of lower than 1% and an ester content, more particularly a methyl acetate content, of lower than 1% is also described, which is provided in the form of flakes a few millimeters in size or of a powder with an average particle size advantageously higher than 600 μm.

BACKGROUND OF THE INVENTION

The object of the present invention is a poly(vinyl alcohol)purification process, as well as a poly(vinyl alcohol) in the form offlakes of a few millimeters in size, or in the form of a powder whoseaverage particle size of greater than 600 μm, and having low methanoland methyl acetate contents.

Poly(vinyl alcohol) is a polymer obtained by reaction of a poly(vinylester), generally poly(vinyl acetate), with an alcohol which can beethanol, or, for an industrial use, methanol. The reaction is carriedout with a catalyst which can be a basic catalyst such as sodiumhydroxide, or even with an acidic catalyst such as sulphuric orhydrochloric acid. The basic catalyst is preferred for industrial use.

When the alcohol (called PVA in what follows) is prepared in adiscontinuous manner, a polymer is obtained as a solid by precipitationin the alcohol used during an alcoholysis, and in the ester releasedduring this same reaction; the precipitation generally takes place in amixer. A step of neutralisation of the residual catalyst is then carriedout, then a washing step of the polymer which is dried later.

When the process is carried out continuously, the product resulting fromthe reaction is first of all placed in the form of flakes mainly in theform of irregular flakes of 2 to 6 mm and of a thickness of a few tenthsof millimeters, by means of a moist grinder, and a step ofneutralisation of the residual catalyst is carried out, followed by awashing step. Upon completion of these operations, the polymer isseparated as a solid which is dried by any known means. Once the productis dried, a grinding of the dried polymer is generally carried out.

Usually, it is noted that the particle size of the PVA particlesobtained by the first process is finer, lower than 200 μm, than with theprocesses carried out continuously. In fact, by the continuousprocesses, the particle size of the particles after grinding is greaterthan 600 μm, even if a priori nothing prevents obtaining a finerparticle size via this route. It is to be noted that particles having alarger size are interesting in this sense that they have betterproperties of use.

It is towards the above-mentioned drying steps that the presentinvention is directed. In fact, whatever the process used, the PVAresulting from the alcoholysis reaction contains a significant amount ofsolvents present during the reaction, such as the alcohol used and theester released. Now, it is necessary to remove these solvents with aview to obtaining contents in the order of a few percent.

However, it is not possible to dry the PVA without precaution since thisproduct is temperature-sensitive. In fact, if it is submitted to a hightemperature, greater than or in the order of 100° C., for a significantperiod of time, it is noted then that some of its characteristicschange, such as the water-solubility, the level of hydrolysis, theviscosity and the coloration, which is to be avoided.

A first obvious process for removing these solvents consists simply inheating the PVA at a temperature lower than that from which the polymerstarts to degrade. However, such a process is not worthwhile sincelowering the temperature leads to significantly increasing the dryingtime. Furthermore, this type of process does not allow sufficientlyremoving the alcohol present.

In order to solve this problem of effectiveness, without going to thedetriment of the polymer's characteristics, it has been proposed tocarry out the purification of the PVA by carrying out a heat treatmentof the latter under an atmosphere having a relative humidity of at least8%, more particularly lower than 15%. Thus, the drying is carried out attemperatures between 90 and 115° C., preferably under vacuum such thatthe speed of removal of the solvents is further accelerated, which isfound to be in the order of a few hours.

However, this process seems more particularly suitable for the drying ofPVA particles of small size. In fact, in this case, diffusion into theinside of the particles is facilitated with respect to particles oflarge size, and from this fact the removal of solvents is renderedeasier. Thus, when this type of drying is carried out on a PVA obtainedby a continuous process, with a relative humidity such as mentionedabove, and at reduced pressure, the resulting PVA has a methanol contentand an ester content of 1%.

Another more recently proposed process consists in mixing the PVA to betreated with liquid water in an amount of 10-30% by weight of water withrespect to the polymer, such that a swelling of the particles isobtained. The particles are then submitted to a heat treatment in avertical drier. The poly(vinyl alcohol) is introduced into the head ofthe drier and passes into a first zone surrounded by a double envelopeheated with water. The temperature in this zone is one of the essentialpoints of this process. It is in fact specified that if this temperaturewas too high, i. e. if the means of heating this zone was vapour and nolonger water, the water would have evaporated before being able to actupon the polymer, as a result, the removal of the solvents would not besufficient. After the passage into this first zone, the PVA is led intotwo other parts, heated by means of a double envelope comprising vapour,such that the water which is trapped in the polymer particles isevacuated.

This process is efficient but the operation time is a few hours,necessitating the implementation of a vertical drier. Furthermore, thisprocess does not allow treating polymers comprising lower than 10% byweight of solvent with respect to the dry weight of PVA.

SUMMARY OF THE INVENTION

The object of the present invention is a poly(vinyl alcohol)purification process which is simple to carry out, economical and whichcan be furthermore rapid.

A further object of the present invention is a poly(vinyl alcohol)having very low solvent contents, and which are in the form of flakeswhose sides measure at the most 10 mm, or in the form of a powder whoseparticle size is high.

Thus, the present invention consists in purifying a poly(vinyl alcohol)by submitting it to a heat treatment with agitation and under anatmosphere unsaturated by water and having a water vapour mass contentbetween 20 and 100%.

The process according to the invention is particularly suitable forobtaining a PVA having an alcohol content of lower than 1% by weightwith respect to the weight of moist PVA, as well as an ester content oflower than 1% with respect to the same reference. More particularly, thealcohol is methanol and the ester is methyl acetate.

Particularly advantageously, the present invention enables obtaining aPVA having an alcohol content of lower than 0.5% by weight with respectto the weight of moist PVA. Advantageously, the invention furtherenables obtaining a PVA with an ester content of lower than 0.5%.

Another object of the present invention is a poly(vinyl alcohol) in theform of flakes whose sides measure at the most 10 mm, and having analcohol content, more particularly a methanol content, of lower than 1%and an ester content, more particularly a methyl acetate content, oflower than 1%, these values being expressed by weight with respect tomoist PVA.

Another object of the present invention is a poly(vinyl alcohol) havingan average particle size of greater than 600 μm and having an alcoholcontent, more particularly a methanol content, of lower than 1% and anester content, more particularly a methyl acetate content, of lower than1%, these values being expressed by weight with respect to moist PVA.

It has been found in a surprising way that the polymer treated under theconditions of the present invention, i. e. notably in the presence ofhigh amounts of water, was not dissolved.

Furthermore, the process according to the invention allows purifyingpoly(vinyl alcohol) such that very low solvent contents be attained, ina treatment time which can be lower than an hour. Furthermore, thisprocess does not necessitate the use of a particular apparatus.

However, other advantages and characteristics of the process accordingto the invention will become evident upon reading the description andExamples that follow.

DETAILED DESCRIPTION OF THE INVENTION

The PVA that can be treated by the process according to the inventioncan have any degree of polymerisation and/or level of alcoholysis.

The process according to the invention is particularly suitable forpurifying PVA having an alcoholysis level of between 70 and 99%, moreparticularly between 95 and 99%.

Furthermore, the process of the invention enables treating the PVAdirectly resulting from the process, after neutralisation of theresidual catalyst. It is even possible to treat, according to theinvention, a PVA which is more or less dried by conventional processes.Thus, as an illustration, a PVA can be treated which has between 3 and80% of solvents, expressed with respect to the weight of moist polymer.

The process according to the invention is suitable for purifying a PVAoriginating from preparative continuous or discontinuous methods.However, the process is particularly well adapted to drying PVAresulting from continuous processes, for which the problem of drying ismore critical due to the significant size of the PVA particles to bedried.

According to a first characteristic of the process according to theinvention, the treatment takes place in the presence of an atmospherecomprising water vapour and having a water vapour mass content ofbetween 20 and 100%. "Water vapour content" is understood as meaning theratio of the mass of water vapour and the total mass of water vapour anddiluting gases.

More particularly, the water vapour mass content is between 40 and 100%.Preferably, the water vapour mass content is between 60 and 100%.

The atmosphere under which the process according to the invention iscarried out comprises water vapour and optionally a diluting gas such asair, nitrogen or even any other gas inert towards PVA under thetreatment conditions.

According to a second characteristic of the invention, the atmosphere isunsaturated by water. The saturation is defined as being the limitbetween a monophasic medium containing water vapour and a biphasicmedium containing both water droplets and water vapour. This definitionis valid for both pure water and for a mixture of water and a dilutinggas.

The atmosphere under which the treatment according to the invention iscarried out can approach saturation without however being attained.

According to a first variant, the treatment is carried out with watervapour and a diluting gas. In this case, the characteristics oftemperature and pressure are selected such that the atmosphere has notattained the saturation limit.

According to a second variant, only water vapour is used. In this case,the vapour is superheated so as not to attain the saturation limit underthe conditions of temperature and pressure.

According to another characteristic of the present invention, theoperation takes place with agitation. More particularly, the agitationis such that the polymer is found in the form of a suspension in thegaseous atmosphere, whose particles are independent.

It is to be noted that it is important to minimise, even to prevent thecondensation of water onto the PVA particles, such that a partialsolubilisation of the PVA is prevented which could lead to the particlessticking together, rendering the particles useless for what follows.

Furthermore, the agitation must preferably be sufficient in order toenable a good solid-gas contact, so as to optimally get rid of thesolvents concerned.

The condensation of a film of water on the particles of PVA can even beprevented by controlling the temperature of the PVA at the start of thetreatment. Thus, the temperature of the product at the start of thetreatment is more particularly around the dew point of the treatmentatmosphere. Preferably, this temperature is higher than the dew point ofthe treatment atmosphere.

Furthermore, it is to be noted that in order to prevent any problem ofdegradation of the polymer, it is preferable that the temperature of thePVA does not go over 130° C. during the treatment.

According to a variant of the invention, the amount of water vapourcoming into contact with the PVA (heated as indicated before) during thetreatment is between 0.2 and 0.8 kg per kilogram of dry PVA. Preferably,this amount is between 0.4 and 0.6 kg with respect to the samereference.

The process according to the invention can therefore take place in anytype of apparatus known to the person skilled in the art which comprisesmechanical means of mechanical agitation. Apparatuses can even be usedin which the agitation is carried out by means of a gas flow, as is thecase notably for fluidised beds, transported beds or even jet beds.

More particularly, the treatment according to the invention is carriedout with agitation for which the Froude number is between 0.04 and 3,and preferably between 0.06 and 2.5.

It is recalled that the Froude number corresponds to the followingratio:

    (π.N).sup.2.D/g

in which formula N represents the number of turns per second, D thediameter of the stirrer and g the acceleration due to gravity.

The time of the process according to the invention is advantageouslyreduced with respect to known prior processes since it is possible toobtain the solvent contents desired in a treatment time as short as aquarter of an hour to an hour. Of course, longer times are not excluded.

In fact, the treatment time can be adapted very advantageously, with thetype of apparatus that is used for drying the PVA.

Thus, according to a first particularly advantageous variant of thepresent invention, the PVA treatment time is between a quarter of anhour and 2 hours, more particularly between half an hour and an hour.According to this variant, the drying is carried out with a strongeragitation, i. e. corresponding to a Froude number between 0.5 and 3, andpreferably between 0.5 and 2.5.

Furthermore, the unsaturated water vapour introduction flow rate is suchthat the amount of vapour placed in contact with the heated product isbetween 0.2 and 0.8 kg per kilogram of dry PVA, and preferably between0.4 and 0.6 kg with respect to the same reference.

According to a second variant of the present invention, the PVAtreatment is carried out for a longer time, 2 to 4 hours for example. Inthis case, the treatment according to the invention can be carried outwith a weaker agitation, i. e. corresponding to a Froude number between0.04 and 0.5, preferably between 0.06 and 0.5.

The unsaturated water vapour introduction flow rate is determined by theperson skilled in the art such that the amount of vapour placed incontact with the heated product be between 0.2 and 0.8 kg per kilogramof dry PVA, and preferably between 0.4 and 0.6 kg with respect to thesame reference.

The pressure at which the operation is carried out is at least 0.5absolute bar. More particularly, it is between 0.5 and 6 absolute bar.Advantageously, the pressure is between 0.8 and 3 absolute bar.

Advantageously, the pressure at which the treatment is carried out isaround atmospheric pressure.

The temperature of the entry of the gases during this step is between 80and 350° C. and depends upon the heating means used.

The heating of the medium during the pre-heating of the PVA or duringthe introduction of the water vapour can be done by any known means.

Thus, a heating of the conductive type can be carried out, by using anapparatus comprising a double envelope. A convective type heating canalso be carried out by means of a hot gas which will advantageously bethe atmosphere under which the treatment is carried out. These two meansof heating can even be combined.

According to a particularly advantageous variant of the presentinvention, a second step can be carried out which consists in submittingthe resulting PVA to a additional heat treatment essentially intended toremove the water present in the PVA from the first step. It is to benoted that this operation further allows even removing a few traces ofsolvents.

This conventional drying operation can be carried out by any means knownto the person skilled in the art. Thus, a conductive drying can becarried out under vacuum or at atmospheric pressure. This step can evenbe carried out by a convective drying by means of a hot gas. This hotgas can be selected from superheated water vapour and/or any other gasinert towards PVA.

This operation can be carried out in a fluidised bed or a crossed bed.

As has been indicated before, the drying process according to theinvention allows decreasing the contents of the solvents, the alcoholand the ester, present in the PVA, without observing degradation of thequality of the PVA.

Thus, the PVA obtained according to the process according to theinvention has an alcohol content of lower than 1% by weight with respectto the weight of moist PVA, as well as an ester content of lower than 1%with respect to the same reference. More particularly, the alcohol ismethanol and the ester is methyl acetate.

Particularly advantageously, the present invention allows obtaining aPVA having an alcohol content of lower than 0.5% by weight with respectto the moist PVA. The present invention also allows obtaining,advantageously, a PVA with an ester content of lower than 0.5%.

It is to be noted that according to the quality of the PVA treated, i.e. notably, according to its degree of drying before the treatment ofthe invention, the ratio, expressed by weight, of alcohol to ester isbetween 1 and 20. More particularly, said ratio is between 1 and 10, andpreferably between 1 and 5.

The object of the present invention is a poly(vinyl alcohol) in the formof flakes whose sides measure at the most 10 mm and having an alcoholcontent, more particularly a methanol content, of lower than 1% and anester content, more particularly a methyl acetate content, of lower than1%, these values being expressed by weight with respect to moist PVA.

The PVA flakes more particularly originate from the continuous PVAsynthetic process and have been the subject of the drying treatmentaccording to the invention.

The dimensions of the sides are more particularly greater than 1 mm andat the most 10 mm. Preferably, the dimension of the sides is between 2and 5 mm. The thickness of the flakes is in the order of 0.05 to 0.3 mm,preferably in the order of 0.1 to 0.2 mm.

Finally, the object of the present invention is a poly(vinyl alcohol)whose average particle size is greater than 600 μm and having an alcoholcontent, more particularly a methanol content, of lower than 1% and anester content, more particularly a methyl acetate content, of lower than1%, these values being expressed by weight with respect to moist PVA.

More particularly, the PVA according to the invention has an averageparticle size between 600 and 1000 μm, preferably the average particlesize is between 700 and 900 μm.

It is to be noted than the PVA described above can notably be obtainedby grinding, by any means known to the person skilled in the art, of theflakes of PVA which have been described before.

According to an advantageous variant of the invention, the PVA, whetherit is in the form of flakes or in the form of a powder whose particlesize is greater than 600 μm, has a methanol content of lower than 0.5%,and a methyl acetate content of lower than 0.5%, contents expressed byweight with respect to the weight of moist PVA.

What has been said about the poly(vinyl alcohol), and notably about thelevel of alcoholysis, remains valid.

Moreover, the PVA according to the invention has a coloration close tothat of he product originating from the synthesis. Non-limiting examplesof the present invention are now given.

EXAMPLE 1

This example illustrates a process of purifying PVA in two successivesteps; the first being carried out in the presence of diluted watervapour.

The apparatus in which the treatment according to the invention iscarried out (turbosphere Moritz®), comprises a rounded tank of maximalworking capacity of 6 liters, externally heated by a double envelope runthrough with water vapour at 130° C.

The envelope is equipped with an agitation means having the same form,for the main part of the working volume, as that of the tank, giving thePVA flakes a significant mixing movement.

The nitrogen and the water vapour can be introduced via the upper lidwith controlled flow rates.

The analyses of the methanol content and the methyl acetate content aredone by "head space" chromatography. The determination of the watercontent is done by means of a Karl-Fischer Metrohm® apparatus with oven.

1489 g of PVA flakes comprising 28% methanol and 30.4% methyl acetateare loaded into the heated apparatus described above.

The water vapour flow rate at atmospheric pressure is 420 g/h and thenitrogen flow rate is 190 g/h.

The water mass content of the atmosphere is therefore 69% for atemperature around 95° C.

The Froude number is 1.4.

The pressure is 1020 mbar.

The temperature of the polymer rose from an initial plateau towards 80°C. up to 107° C. after one hour of introduction of vapour and nitrogen.

After one hour of treatment, the product is dried, in a second step, byconduction through the walls of the apparatus. The flow rate of nitrogenis maintained at the same flow rate as before.

The operation is conducted for one hour.

The pressure is 1020 mbar.

Upon completion of the drying step, the product is in the form ofindividual and brittle flakes of very light ivory colour.

The product is ground to 800 μm and has the same contents of methanol,methyl acetate and water as the flakes.

The results are grouped in the Table below:

    ______________________________________                                        time     methanol     methyl acetate                                                                           water                                        (minutes)                                                                              (weight %)   (weight %) (weight %)                                   ______________________________________                                         0       28           30.4       --                                            60      0.65         0.76       4.5                                          120      0.39         0.75       0.95                                         ______________________________________                                    

The contents in weight % are expressed with respect to the weight ofmoist PVA.

EXAMPLE 2

This example illustrates a process of treating PVA in two successivesteps the first of which is carried out in the presence of slightlysuperheated pure water vapour.

The treatment is carried out in the same apparatus as that described inExample 1.

1500 g of PVA comprising 16.2% methanol and 1.4% methyl acetate areintroduced into the heated apparatus.

The pure water vapour (water mass content of 100%) is introduced with aflow rate of 1600 g/h at a temperature around 102° C. for 40 minutes.

The operation is carried out under a pressure of 1020 mbar.

The Froude number is 2.1.

The apparatus is then placed under vacuum of 100 mbar for one hour.

The temperature of the product rises from 85 to 107° C. during thetreatment of the water vapour attaining 105° C. at the end of thedrying.

The results obtained are grouped in the following Table:

    ______________________________________                                        time     methanol     methyl acetate                                                                           water                                        (minutes)                                                                              (weight %)   (weight %) (weight %)                                   ______________________________________                                         0       16.2         1.4        --                                           30       0.4          0.07       10.2                                         40       0.08         0.01       12.8                                         100      0.03         0.03        0.32                                        ______________________________________                                    

It is noticed that as soon as 30 minutes, the treatment with the watervapour allows attaining a methanol content and a methyl acetate contentof lower than 0.5%. Despite the water content being 12.8% beforestarting the drying step, the PVA resulting from the process accordingto the invention is in the form of individual and dry particles.

The final product is constituted of individual flakes of very lightivory colour.

The product was ground to 800 μm and has the same contents of methanol,methyl acetate and water as the flakes.

EXAMPLE 3

This example illustrates a one-step process in the presence of dilutedwater vapour.

881 g of PVA titrating 5.1% methanol and 0.73% methyl acetate areintroduced into the hot apparatus described in Example 1.

A mixture of gases having a water mass content of 69% and comprising 190g/h of nitrogen and 420 g/h of water vapour, is introduced atatmospheric pressure and at a temperature in the order of 95° C. for 3hours.

The temperature of the product attains 112° C. at the end of the step.

The Froude number is 2.1.

The results are grouped in the Table below:

    ______________________________________                                        time     methanol     methyl acetate                                                                           water                                        (minutes)                                                                              (weight %)   (weight %) (weight %)                                   ______________________________________                                         0       5.1          0.73       --                                           30       1.46         0.46       --                                           60       0.55         0.27       2                                            90       0.42         0.28                                                    120      0.29         0.22       1.9                                          180      0.18         0.20       1.8                                          ______________________________________                                    

The product is in the form of dry, brittle flakes and keep their initialcolour.

The product was ground to 800 μm and has the same contents of methanol,methyl acetate and water as the flakes.

COMPARATIVE EXAMPLE 4

This example consists in carrying out a treatment in the presence ofwater vapour under reduced pressure.

962 g of PVA comprising 5.1% methanol and 2.1% methyl acetate are loadedinto the hot apparatus described in Example 1.

Water vapour is introduced with a flow rate of 300 g/h to a temperatureof 102° C. (mass content of 100%) at a pressure of 300 absolute mbar for95 minutes.

The Froude number is 2.1.

The temperature of the product rises to 105° C. attaining a stable valueof 114° C.

The following Table groups the results obtained:

    ______________________________________                                        time     methanol     methyl acetate                                                                           water                                        (minutes)                                                                              (weight %)   (weight %) (weight %)                                   ______________________________________                                         0       5.1          2.1        --                                            5       4.48         2.02       1.96                                         65       1.44         1.49       0.64                                         95       1.12         1.47       0.48                                         ______________________________________                                    

Despite a vapour treatment of 95 minutes, the residual contents of thesetwo solvents are greater than 1%, when such conditions are put intooperation.

The flakes obtained are dry and brittle.

The product was ground to 800 μm and has the same contents of methanol,methyl acetate and water as the flakes.

COMPARATIVE EXAMPLE 5

The object of this example is a conventional type drying.

1538 g of PVA comprising 28% methanol and 30.4% methyl acetate areloaded into the hot apparatus.

190 g/h of nitrogen (water mass content of 0) under atmospheric pressure(1020 mbar) are used throughout the test.

The Froude number is 1.4.

The temperature of the product rises from 82° C. to 110C.

The following Table groups the results obtained:

    ______________________________________                                        time          methanol  methyl acetate                                        (minutes)     (weight %)                                                                              (weight %)                                            ______________________________________                                         0            28        30.4                                                  30            10.35     2.64                                                  60            3.05      2.54                                                  120           1.39      2.43                                                  ______________________________________                                    

It is noted that despite a treatment time of 2 hours, the methanolcontent and the methyl acetate content remain greater than 1%.

The product was ground to 800 μm and has the same contents in methanol,methyl acetate and water as the flakes.

Furthermore, and in contrast to what is observed when the processaccording to the invention is carried out, a variation in the colorationof the treated PVA is noted, which goes from light ivory toamber-yellow.

What is claimed is:
 1. In a process for purification of poly(vinylalcohol) obtained by a reaction of a poly(vinyl ester) with an alcoholwith release of an ester, the improvement comprising subjecting saidpoly(vinyl alcohol) to a heat treatment with agitation and under anatmosphere having a water vapor content between 20 and 100 wt % andwhich is not saturated with water, to obtain a poly(vinyl alcohol)having a content of said alcohol of less than 1 wt % and a content ofsaid ester of less than 1 wt %, with respect to moist poly(vinylalcohol).
 2. According to the process claim 1, wherein the treatment iscarried out under an atmosphere having a water vapour content between 40and 100 wt %.
 3. The process according to claim 1, wherein the treatmentis carried out under an atmosphere further comprising a diluting gas. 4.The process according to claim 3, wherein the diluting gas is nitrogen,air or any other gas inert towards poly(vinyl alcohol) under treatmentconditions.
 5. The process according to claim 1, wherein the treatmentis carried out under an atmosphere of superheated pure water vapour. 6.The process according to claim 1, wherein the treatment is carried outwith a gas entry temperature between 80 and 350° C.
 7. The processaccording to claim 1, wherein the treatment is carried out with apoly(vinyl alcohol) having, at the start of treatment, a temperaturearound the dew point of the treatment atmosphere.
 8. The processaccording to claim 1, wherein the treatment is carried out with anamount of water vapor coming into contact with the poly(vinyl alcohol)during the treatment of between 0.2 and 0.8 kg per kilogram of drypoly(vinyl alcohol).
 9. The process according to claim 7, wherein thetreatment is carried out at a pressure of at least 0.5 absolute bar. 10.The process according to, claim 1, further comprising subjecting thepoly(vinyl alcohol) obtained to an additional heat drying treatment.